The format specifier %a for printf() in C

Question

I am reading a C language book, it said %f, %e, %g, %a were printf chars used for float and double data types. Currently I can understand %f, %e, %g completely.

When do I need use %a to print float and double type data ?

Can you please show me an example.

Answer 1

The %a formatting specifier is new in C99. It prints the floating-point number in hexadecimal form. This is not something you would use to present numbers to users, but it's very handy for under-the-hood/technical use cases.

As an example, this code:

printf("pi=%a\n", 3.14); 

prints:

pi=0x1.91eb86p+1 

The excellent article linked in the comments explains that this should be read "1.91EB86₁₆ * 2¹" (that is, the p is for power-of-two the floating-point number is raised to). In this case, "1.91EB86₁₆" is "1.5700000524520874₁₀". Multiply this by the "2¹", and you get "3.140000104904175₁₀".

Note that this also has the useful property of preserving all bits of precision, and presenting them in a robust way. For instance you could use this to serialize floating point numbers as text, and not have to worry about repeating/infinite decimals.

Also note that strtod() can convert floating point numbers in hexadecimal form back to actual numbers. Not 100% sure about sscanf() and friends, the documentation was not very clear and I've never used that.